Output stages with gain Enhance Slew rate - Page 2 - diyAudio
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Old 30th April 2003, 03:35 AM   #11
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Michael,

Kindly let me have a copy of your article in Electronics World.

Many thanks,
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Old 30th April 2003, 05:15 AM   #12
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Yeah, I think that I missed the point, too.

Amps with gain in the output stage are a pain to stabilize. And most people know better than to make one. So what is the point?

Unless the point is that anything Prof. Cherry does is either impractical or just plain stupid.

Or both.


Jocko
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Old 30th April 2003, 07:09 AM   #13
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Quote:
Originally posted by Jocko Homo
Yeah, I think that I missed the point, too.

Amps with gain in the output stage are a pain to stabilize. And most people know better than to make one. So what is the point?

Unless the point is that anything Prof. Cherry does is either impractical or just plain stupid.

Or both.


Jocko

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Old 30th April 2003, 07:20 AM   #14
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Smile ...and...

...more examples......download manuals and/or schematics in pdf:


http://www.a-and-t-labs.com/products.htm


...and then of course we have Hafler's designs:


http://www.hafler.com/home/

(click on 'library archive' )

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Old 30th April 2003, 10:26 AM   #15
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Default ...to continue from post#1...

..alternatively,...you may simply multiply the slew rate of gain block G1 by the output stage closed loop gain to obtain the maximum slew rate at the output...thus:
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Old 30th April 2003, 10:31 AM   #16
Nicke is offline Nicke  Sweden
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Walt Jung built an amplifier with gain in the output stage in this article:Walt Jung´s AD823/812 amplifier

I guess he knows what he is doing.

Nicke
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Old 30th April 2003, 03:41 PM   #17
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Default Pardon the redundancy.....

But what is the point?

Slew rate is vastly overrated to begin with.

The easiest way to increase slew rate is degeneration in the input stage to lower its transconductance.

Something the Prof. Cherry doesn't believe in. Must be why he designs theoretical amps that are "impractical". (Your words, not mine. Mine are harsher.)

Jocko
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Old 30th April 2003, 04:21 PM   #18
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Default Re: Pardon the redundancy.....

Quote:
Originally posted by Jocko Homo
But what is the point?

Slew rate is vastly overrated to begin with.

Correct.....

Quote:
Originally posted by Jocko Homo

The easiest way to increase slew rate is degeneration in the input stage to lower its transconductance.
...up to a point...yes...Overdo it though, and you have intolerable offsets at output...more input noise...etc.....

.....Of course it would be idle to pretend a 400V/uS slew rate is desirable......you would have to generate a peak swing in excess of 3KV, (>560KW into 8Ohms), for this to be the case...........

However, i thought it might be a good idea to share one side effect of output stages with gain that you will not find written about anywhere.....at least as far as i know....

Moreover, the first gain stage encapsulating the main compensation network need not necessarily be an expensive discrete circuit, with sub-optimal, high-voltage transistors, (read mediocre Ft),.....a simple high quality, but comparatively inexpensive op-amp., (e.g: AD8610, OPA627, AD797), would be entirely acceptable.

Quote:
Originally posted by Jocko Homo

Something the Prof. Cherry doesn't believe in. Must be why he designs theoretical amps that are "impractical". (Your words, not mine. Mine are harsher.)

Jocko

...you would have to show that Cherry hasn't infact built his 'theoretical' designs....which is an entirely different order of things from being merely 'impractical'....... otherwise this is idle speculation.......
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Old 30th April 2003, 05:02 PM   #19
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Default Where to start.......

Maybe if you had stated that the purpose was to show how one can design with an op-amp, and then an output stage with gain; then maybe my inbox would not be full of confused questions directed at me (intead of you!).

And since you can do that, then you understand why Walt Jung is expected to design such objects.

(If it makes you feel any better, I have worked with a lot of the items you reference.)

Maybe you should more closely read the old stack of AES Journals that I suspect you might have. Marshall Leach ahs written articles explaining input degeneration, and if his advice is followed, it will not have the problems that you allude to.

And if you do read them, then you will see pompous twits like Prof. Cherry opine piously on how stupid Prof. Leach is.

No one that I know suggests using sub-optimal parts. That is your assumption.

Prof. Cherry claims to have built his amps, but then he uses terms like "non plus ultra" to descibe them. Yeah, they measure low distortion, and his awe struck studants may be agog at it, but does he make his living designing and building amps?

No, he doesn't. So his drivel is merely noise to my ears.

Jocko
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Old 30th April 2003, 05:14 PM   #20
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Default Arm chair engineering

First off, Slew rate:

From http://www.passdiy.com/pdf/a40.pdf by Nelson Pass

The combination produced phenomenal high frequency power which hit -25dB (reference peaks below 20kHz) at 50kHz. If the amplifier were clipped, I measured the 30V/us recovery, but with unclipped performance, the highest measured slew was
1.5V/us.

In other recordings and with other cartridges, .5V/us peak slew was more common and was generally associated with cymbals and synthesizers but not piano or vocal material. I found this interesting as I had always imagined higher slews than I found. It certainly makes one wonder whether the value of high
slew rate specifications isn't like damping factor, where very high values
show diminishing returns in sonic performance.

Second........
Walt Jung built an amplifier with gain in the output stage in this article: Walt Jung´s AD823/812 amplifier. This is not even a power amplifier but is a composite op amp preamp circuit for preamp line level use. Both the op amps use unity gain output stages if I am not mistaken.

Last and least.......

If people on going to provide links, please link what you are referring to so one doesn't have to wade through the web site to see what you found of interest For example:

http://www.hafler.com/techsupport/index.asp?ID=3 and tell us which of the couple dozen manuals you are referencing.

http://www.a-and-t-labs.com/K6_Sw_Am...atic/k6amp.pdf

(This amp has some design features that are a big enough liabilities to make the topology selection pretty irrelevant. This is based on actual experience and not schematic browsing by the way.)

The "I wrote a Spice model or drew a schematic" amplifier design school is now be reduced to include block diagrams! Everyone raise their hand that has actually built a real amplifier. I am reminded of the news "analysis" programs where a bunch a journalist sit around and speculate over issues that they have 1% of the information about.

No mention of the Nelson Pass designs which are some of the best amps with gain around the output stage?
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